1. Field of the Invention
The present invention relates to an ink jet application device and an ink jet application method for spaying ink-droplets on a substrate.
2. Description of the Related Art
Recently, a demand for a flat display increases with the development of a personal computer (PC), a portable phone, etc. A liquid crystal display, an organic EL (electro-luminescence) display, etc. are employed as the flat display. In a process for manufacturing the flat display, a luminescent layer material (ink) is applied on a surface of a substrate by an application head mounted in an ink jet application device.
As shown in FIG. 1, an application head 101 comprises nozzles 103a, 103b, 103c, ink chambers 105a, 105b, 105c, a channel unit 107, a diaphragm 109 and actuators 111a, 111b, 111c. The ink chambers 105a, 105b, 105c are communicated with the nozzles 103a, 103b, 103c, respectively. The channel unit 107 accommodates the ink chambers 105a, 105b, 105c. The diaphragm 109 closes open ends of the ink chambers 105a, 105b, 105c. The actuators 111a, 111b, 111c are disposed at the open end sides of the ink chambers 105a, 105b, 105c, respectively.
When voltages are applied to the actuators 111a, 111b, 111c, the actuators 111a, 111b, 111c are deformed to vary volumes of the ink chambers 105a, 105b, 105c via the diaphragm 109. If the actuators 111a, 111b, 111c shrink, ink is supplied to the ink chambers 105a, 105b, 105c. In contrast, if the actuators 111a, 111b, 111c expand, ink is sprayed from the ink chambers 105a, 105b, 105c. 
Uniforming brightness of the luminescent layer material across a display area is cited as one of characteristics required for the flat display. In a case where ink is sprayed on the substrate by means of the ink jet application device, in order to uniform the brightness of the luminescent layer material, it is necessary to substantially uniform a shape of the ink across the display area after drying. Therefore, a volume of an ink-droplet is precisely controlled in each pixel of the flat display.
However, in a case where there are variations of an actuator characteristic, a nozzle shape and a volume of an ink-droplet, even if voltages applied to the actuators 111a, 111b, 111c are equal to one another, the volume of the ink-droplet sprayed by the nozzles 103a, 103b, 103c is not uniformed (see FIG. 2). It is here noted that the variation of the volume of the ink-droplet is generated by an air bubble or invaded dust in the ink chamber.
In order to solve the above-described problem, as shown in FIG. 3, specific voltages are supplied to the actuators 111a, 111b, 111c via a switching circuit 115 by dividing a power supply voltage from a voltage supply (not shown) with a resistance dividing circuit 113 consisting of a variable resistance VR1 and a fixed resistance R1, on the basis of a spray signal from a spray order portion 117. An operator manually adjusts the variable resistance VR1 while measuring a volume of an ink-droplet with his/her eye.
When the number of nozzles is small, the operator can easily control the volume of the ink-droplet by adjusting the variable resistance VR1 manually. However, when the number of nozzles is larger, it takes a lot of time and trouble to control the volume of the ink-droplet by adjusting the variable resistance VR1 manually. In fact, when three application heads each having 64 nozzles are mounted in an ink jet application device and then a volume of an ink-droplet is controlled, it takes hours to control the volume of the ink-droplet while it takes a few minutes to apply ink on a substrate.
Further, since the variable resistance VR1 is manually adjusted, a variation of a volume of an ink-droplet is easily generated. This makes it more difficult to stably remain quality of a flat display.